There is currently considerable interest in the production of ceramic materials such as zirconia for use in a variety of technologically advanced applications. In most cases it is required that the starting material for a ceramic article be a powder of very fine particle size.
The grinding of powders to a fine size is a technology which has been highly researched, the object being to achieve for any given application the most efficient grinding, that is, the finest possible particle size in the shortest possible time with the least possible input of energy. The best possible grinding for a given material usually involves a compromise in one or more of these requirements.
Grinding means may be conveniently divided into two types, high energy types and low energy types. The former types include sand, bead and shot mills, and the latter include agitated media mills such as "attritors".
In high energy mills, efficient grinding requires that there be sufficient "stand-off" distance between the particles of grinding media. This is well described in the authoritative "Paint Flow and Pigment Dispersion" by Temple C. Patton (2nd Ed., Wiley, 1979) at pages 451-452. Here Patton concludes that an average stand-off distance of 60 um is required for optimum results.
Low energy grinding means are not restricted in this manner and a wide range of proportions of grinding media/fluid/material to be ground can be achieved. Patton (at page 440) states that there is little point in using grinding media of size lower than about 3 mm in such an apparatus as efficiency drops and grinding times become excessively long. There have, however, been experiments with smaller grinding media in attrition mills. For example, Mankosa, Adel and Yoon describe the use of small (1.6 mm) media in the attrition milling of coal ("Powder Technology", 49 (1986), 75-82). However, this reference did not record the attainment of any particle size below 2 um.